ENH: Make signed/unsigned integer comparisons exact

This makes comparisons between signed and unsigned integers exact
by special-casing promotion in comparison to never promote integers
to floats, but rather promote them to uint64 or int64 and use a
specific loop for that purpose.

This is a bit lazy, it doesn't make the scalar paths fast (they never were
though) nor does it try to vectorize the loop.
Thus, for cases that are not int64/uint64 already and require a cast in
either case, it should be a bit slower.  OTOH, it was never really fast
and the int64/uint64 mix is probably faster since it avoids casting.

---

Now... the reason I was looking into this was, that I had hoped
it would help with NEP 50/weak scalar typing to allow:

    uint64(1) < -1  # annoying that it fails with NEP 50

but, it doesn't actually, because if I use int64 for the -1 then very
large numbers would be a problem...
I could probably(?) add a *specific* "Python integer" ArrayMethod for comparisons
and that could pick `object` dtype and thus get the original Python object
(the loop could then in practice assume a scalar value).

---

In either case, this works, and unless we worry about keeping the behavior
we probably might as well do this.
(Potentially with follow-ups to speed it up.)

3 files changed, 76 insertions, 10 deletions

diff --git a/numpy/core/src/umath/loops.c.src b/numpy/core/src/umath/loops.c.src
index 397ebaca2..97a74b425 100644
--- a/numpy/core/src/umath/loops.c.src
+++ b/numpy/core/src/umath/loops.c.src
@@ -545,6 +545,48 @@ NPY_NO_EXPORT void
 /**end repeat1**/
 /**end repeat**/
 
+
+/*
+ * NOTE: It may be nice to vectorize these, OTOH, these are still faster
+ *       than the cast we used to do.
+ */
+
+/**begin repeat
+ * #kind = equal, not_equal, less, less_equal, greater, greater_equal#
+ * #OP = ==, !=, <, <=, >, >=#
+ */
+NPY_NO_EXPORT void
+LONGLONG_Qq_bool_@kind@(char **args, npy_intp const *dimensions, npy_intp const *steps, void *NPY_UNUSED(func))
+{
+    BINARY_LOOP {
+        const npy_ulonglong in1 = *(npy_ulonglong *)ip1;
+        const npy_longlong in2 = *(npy_longlong *)ip2;
+        if (in2 < 0) {
+            *(npy_bool *)op1 = 0 @OP@ in2;
+        }
+        else {
+            *(npy_bool *)op1 = in1 @OP@ (npy_ulonglong)in2;
+        }
+    }
+}
+
+NPY_NO_EXPORT void
+LONGLONG_qQ_bool_@kind@(char **args, npy_intp const *dimensions, npy_intp const *steps, void *NPY_UNUSED(func))
+{
+    BINARY_LOOP {
+        const npy_longlong in1 = *(npy_longlong *)ip1;
+        const npy_ulonglong in2 = *(npy_ulonglong *)ip2;
+        if (in1 < 0) {
+            *(npy_bool *)op1 = in1 @OP@ 0;
+        }
+        else {
+            *(npy_bool *)op1 = (npy_ulonglong)in1 @OP@ in2;
+        }
+    }
+}
+/**end repeat**/
+
+
 /*
  *****************************************************************************
  **                           DATETIME LOOPS                                **
diff --git a/numpy/core/src/umath/loops.h.src b/numpy/core/src/umath/loops.h.src
index ab54c1966..cce73aff8 100644
--- a/numpy/core/src/umath/loops.h.src
+++ b/numpy/core/src/umath/loops.h.src
@@ -211,6 +211,17 @@ NPY_NO_EXPORT void
 /**end repeat1**/
 /**end repeat**/
 
+/**begin repeat
+ * #kind = equal, not_equal, less, less_equal, greater, greater_equal#
+ * #OP = ==, !=, <, <=, >, >=#
+ */
+NPY_NO_EXPORT void
+LONGLONG_Qq_bool_@kind@(char **args, npy_intp const *dimensions, npy_intp const *steps, void *NPY_UNUSED(func));
+NPY_NO_EXPORT void
+LONGLONG_qQ_bool_@kind@(char **args, npy_intp const *dimensions, npy_intp const *steps, void *NPY_UNUSED(func));
+
+/**end repeat**/
+
 
 #ifndef NPY_DISABLE_OPTIMIZATION
     #include "loops_unary.dispatch.h"
diff --git a/numpy/core/src/umath/ufunc_type_resolution.c b/numpy/core/src/umath/ufunc_type_resolution.c
index 12187d059..decd26580 100644
--- a/numpy/core/src/umath/ufunc_type_resolution.c
+++ b/numpy/core/src/umath/ufunc_type_resolution.c
@@ -381,8 +381,28 @@ PyUFunc_SimpleBinaryComparisonTypeResolver(PyUFuncObject *ufunc,
             if (out_dtypes[0] == NULL) {
                 return -1;
             }
-            out_dtypes[1] = out_dtypes[0];
-            Py_INCREF(out_dtypes[1]);
+            if (PyArray_ISINTEGER(operands[0])
+                    && PyArray_ISINTEGER(operands[1])
+                    && !PyDataType_ISINTEGER(out_dtypes[0])) {
+                /*
+                 * NumPy promotion allows uint+int to go to float, avoid it
+                 * (input must have been a mix of signed and unsigned)
+                 */
+                if (PyArray_ISSIGNED(operands[0])) {
+                    Py_SETREF(out_dtypes[0], PyArray_DescrFromType(NPY_LONGLONG));
+                    out_dtypes[1] = PyArray_DescrFromType(NPY_ULONGLONG);
+                    Py_INCREF(out_dtypes[1]);
+                }
+                else {
+                    Py_SETREF(out_dtypes[0], PyArray_DescrFromType(NPY_ULONGLONG));
+                    out_dtypes[1] = PyArray_DescrFromType(NPY_LONGLONG);
+                    Py_INCREF(out_dtypes[1]);
+                }
+            }
+            else {
+                out_dtypes[1] = out_dtypes[0];
+                Py_INCREF(out_dtypes[1]);
+            }
         }
         else {
             /* Not doing anything will lead to a loop no found error. */
@@ -398,15 +418,8 @@ PyUFunc_SimpleBinaryComparisonTypeResolver(PyUFuncObject *ufunc,
                 operands, type_tup, out_dtypes);
     }
 
-    /* Output type is always boolean */
+    /* Output type is always boolean (cannot fail for builtins) */
     out_dtypes[2] = PyArray_DescrFromType(NPY_BOOL);
-    if (out_dtypes[2] == NULL) {
-        for (i = 0; i < 2; ++i) {
-            Py_DECREF(out_dtypes[i]);
-            out_dtypes[i] = NULL;
-        }
-        return -1;
-    }
 
     /* Check against the casting rules */
     if (PyUFunc_ValidateCasting(ufunc, casting, operands, out_dtypes) < 0) {